Optical measurement systems typically employ one or more photodetectors to detect incident light and then use information derived from the detected light for various purposes. For example, a photodetector incorporated into a digital camera (in the form of an image sensor) can be used to measure light intensities associated with various objects in a scene to be captured by the digital camera. Digital cameras generally include various exposure settings that can be used to address various ambient lighting conditions. The exposure settings can be suitably adjusted after an initial measurement of ambient light is carried out. The initial measurement may be carried out either by using circuitry contained in the digital camera itself or by using an external light meter. However, such ambient light measurements often prove to be rough approximations that do not accurately reflect the actual amount of ambient light that may be present at a particular moment in time when the camera is used to capture an image of an object located at a distance far from the camera.
In some applications other than photography, ambient light measurements can be carried out using various other techniques and procedures. However, in many instances, even the use of these other techniques and procedures fails to provide satisfactory results. For example, ambient light measurement circuitry used in some traditional time-of-flight optical distance measurement systems for measuring ambient light and addressing resulting adverse effects often proves inadequate and less than optimal. This shortcoming may be attributable, at least in part, to the more complex nature of the distance measurement procedure in comparison to various light detection procedures employed in digital cameras, for example.
As is known, a time-of-flight (TOF) optical distance measurement system operates by transmitting a beam of light towards a target object and then waiting to receive a reflected portion of the emitted light after reflection by the target object. The time delay between transmission of the light beam and reception of the reflected light is used to calculate the distance between the measurement system and the target object. Such TOF algorithms are known as direct TOF systems because the distance calculation is based directly on the time delay, i.e., the time of flight. In indirect TOF systems, the distance calculation is not directly calculated from the time delay, but is determined indirectly from the phase difference between the transmitted beam and the received reflected beam.
Understandably, the amount of reflected light can be very small in comparison to the amount of ambient light that may be present in the vicinity of the optical distance measurement system. Existing TOF optical distance measurement systems attempt to eliminate the effects of the ambient light with limited success due in part to complexities related to determining an optimal length of time (sampling period) that can be used for detecting an amount of ambient light with a satisfactory level of accuracy. An excessively long sampling period can lead to undesirable measurement delays with no guarantees that the ambient light will remain unchanged at the moment when the reflected light actually reaches a detector at a later instant in time. On the other hand, a short sampling period can lead to an improper measurement of the ambient light level. The above-referenced U.S. application Ser. No. 14/815,266 is directed to a variety of solutions for selecting the appropriate sampling period that are well suited for use in TOF optical distance measurement systems.
In addition to the robustness of the TOF optical distance measurement system against ambient light, receiver sensitivity, dynamic range and signal-to-noise ratio (SNR) are important performance parameters. For measurement systems that use small measurement spot sizes, laser eye safety requirements limit the allowed average output power of the light transmitting device, which is typically a laser diode. In order to meet certain system performance parameters relating to, for example, measurement range, measurement speed, remission and reflection properties of the measurement target, measurement precision, and the size of the receiver optics, the effective receiver SNR should be maximized in order to achieve high receiver sensitivity. At the same time, the dynamic range of the measurement system is an important performance parameter. The measurement device should be capable of accepting very high signal strength of a nearby highly-reflecting object, which requires the measurement device to have high overload thresholds. However, other important performance parameters such as measurement speed (e.g., frame rate) and signal linearity can be compromised by the use of signal amplification or noise damping measures.
It is therefore desirable to provide an optical measurement system that provides robust ambient light suppression, that has a high SNR, that has high receiver sensitivity, and that is capable of operating over a wide dynamic range.